Turbine buckets (turbine blades) have thin surface coatings of alloy materials to protect the bucket from the detrimental effects of high temperatures and corrosive environments. It is desirable to measure the surface coating thickness and the presence of cracks in the coating. The coating thickness varies to a small degree on buckets and other metal (or semiconductive material) components. Craze cracks that develop in the coating may penetrate into the bucket substrate material. It is important to know whether a crack or other surface flaw has penetrated the surface coating into the underlying bucket substrate material. Accordingly, accurate measurements of the surface thickness coating are needed to determine if a crack or flaw has propagated into the substrate material.
The present invention provides an apparatus for thermoelectric scanning of a turbine bucket to measure the thickness of the surface coating on the bucket. The apparatus in one embodiment is a thermoelectric potential probe having a rolling-ball probe head contacting a surface of a test object. The probe head may be substantially spherical and has at least one internal thermal heating element. The probe head also includes a sensor, such as a thermocouple, for detecting a thermoelectric potential signal from the surface of the bucket. The probe head may be alternatively referred to as a thermoelectric potential probe or thermoelectric probe. A tubular axle is fixedly attached to the probe head for rotating the probe head. A rotational encoder may be connected to the axle for generating electrical signals indicative of rotational movements of the axle. In addition, a probe housing may be used to support and provide a bearing surface for the axle.